aberration, and leads to a deterioration in the quality of the image, and hence, resolution. It is important to note, that this
perfect objectivehas never been achieved, although modern plan-apochromats come very close. Before this paradigm shift, simple microscopes using single lenses of various types showed less distorted images than compound instruments at the same magnification, because the multiple lenses in compound microscopes multiplied the aberrations. This web page is an attempt to illustrate the various types of microscope objectives found with compound microscopes and review their evolution over time. We start our discussion in the 1700s, the period from which time onward, a number of examples are still extant.
2on the threaded end, indicating that it was the second to highest power of a set for the microscope it was made for. When viewed under a stereo microscope, this lens has some scratches and scuffs on both surfaces. This pre-achromatic objective produces fair images, but with expected chromatic and spherical aberration.
the trade, and were ground by hand by people, sometimes children, working from home, so tolerances could be wide, and the surface often fraught by imperfections. Worse still, as the lens was loose (some rattle when handled), centering was poor, which all led to an increase in aberrations. These were not well understood, and many observers, using these objectives, believed they saw actual structures that were, in actuality, nothing more than severe distortions that did not represent the actual structures observed. Because of the high magnification, uncorrected for aberrations, many objects appeared to be nothing but vague round blobs. Some observers, such as Milne-Edwards went so far as to believe that all objects were composed of "globules". Today we refer to magnifications which show no more fine detail as empty magnification.
between-lensplaced at the top of the snout carrying the single lens objective, whereby this lens, which was often 4 or 5 inches in focus, functioned as the back lens for the whole series of objectives. This reduced the working distance of the objective, and effectively brought a slight improvement in its angle of aperture.
this instrument was made by that acute and distinguished artist, Mr. Adie, of Edinburgh,.., and features a large oval mirror, as well as an under stage condensing lens. The objective is obviously stopped down to a lesser extent, than was usual at the time, as Goring asserts that it provides
an abundance of lightwhen viewing opaque objects.
where the rays cross each other. The back lens (with a focal length between 3 to 2 up to 2 to 1 times the focal length of the front lens) is placed closely above the stop. Goring claims that:
these object-glasses I can confidently recommend as greatly superior to those in common use; they are bright, clear, and distinct, free from spherical aberration, and will shew no sensible colour with opaque objects of any kind.., but:
When, however, they are made to view an object illuminated from behind, which does not suffer the light to pass through it while its edges are seen, as for example the legs of some insects, some kinds of moss, &c, which have little transparency, the uncorrected colour is then decidedly seen.Goring reports, that this type of objective only works well up to focal lengths of up to 1/4 inch, and had a number of
silver cupsmade
for holding very deep single lenses (of shorter focal length) intended to view opaque objects, which, together with the object-glasses before-mentioned, were executed for me by Mr. Tuther, optician, in High Holborn..(see illustration, right). By now, Goring had obviosly moved back to London, as he also states that:
Mr. Varley and Mr. William Tulley, of Islington, are the only individuals who can make such deep lenses as they ought to be made. Around this time, he also meets Andrew Pritchard, with whom, for awhile, he worked on developing Jewel lenses to be used as simple microscopes, before -in 1825- commissioning William Tulley to make him
a triple achromatic lens of 0.33 inch focus, which heralds the true beginning of the achromatic period. Goring's non-achromatic object glass for low powers is mentioned as late as 1837 in Sir David Brewster's Treatise on the Microscope, with the addition:
Mr Pritchard remarks, that when the focal length of the lens A (front lens) is not less than half an inch, this combination has been employed with considerable advantage, both as regards distinctness and aperture.
Smith's Quarters. Later J.B. Dancer followed suit and also supplied this type of objective. The number 22 on this objective refers, unusually, to the serial number of the microscope to which this objective belongs.
Uncoveredand
Coveredshown on the objective, which is rather inprecise. Many users often employed the
trial and errormethod, whereby with one hand on the correction collar, and the other on the fine adjustment control, the best setting was found which showed the best definition of a given object. Another method, advocated by Andrew Ross, and endorsed by Hugh Powell (in Description of the newly constructed achromatic microscope, 1842), was:
While this method works well for objectives with a longer working distance such as a 1/4 inch, in practice, those with a shorter focal length can not be focused on the object while in the uncovered position,as coverglass thickness prevents this, and the opposite procedure is required: set the correction on fully covered, focus on the top of the coverslip, and then turn the correction collar down, i.e. a bit more towards the uncovered position in order to bring the object into focus. One finds that with objectives of a very short focal length and working distance, the object can still not be brought into focus even with the correction collar in the maximum covered position. These objectives were made to be used with extra thin cover glasses, such as those which Powell & Lealand commissioned from Messrs Chance, which reportedly were only 0.05 mm thick.To place it at the point for viewing objects uncovered, which will be known by observing that the circular line under the word uncovered corresponds with the fixed line; or:, the more ready way is, to adjust it down as far as it will allow, as we always make them to stop at the corrected point. Bring the object into focus by adjustment of the body, then adjust the object-glass till the upper surface of the glass which covers the object is in focus; this can very readily be done while the person is observing, by taking between finger and thumb the milling on the object-glass, and turning it to the left; then bring the object again into focus by the body, and the adjustment is perfect
Coveredand for
Wet, the latter indicating water immersion, and the angular aperture of 135o, which corresponds to a numerical aperture of 1.23 when used as an immersion lens in water, or 0.924 in air-its not clear which this was referring to.
balsam angleor B.A. for the angular aperture of an immersion objective in balsam and this was sometimes inscribed on the objectives of the time, and for some time afterwards, as seen on the Herbert Spencer Objective shown to the left. By 1870, Zeiss, with the help of Abbe, was making all their objectives by calculation; in 1873, the year that Abbe published his scientific work on the resolution of objectives relating to numerical aperture, Tolles made his famous 1/10 inch homogenous immersion objective with a N.A. of 1.25. But Balsam was hard to work with and hardened with time and so was not ideal. Tolles made a glycerine immersion objective that same year. In 1877 Abbe, after testing hundreds of liquids, discovered that Cedarwood oil has a refractive index(1.516) close to glass, and all his oil immersion lenses were then designed to work with this; within a few years Cedarwood Oil replaced softened Canada balsam for all oil immersion objectives.
funnel stopfor an ordinary objective, or later,an iris diaphragm built in to the objective. These tiny iris diaphragms are difficult to make and were not common in objectives until the middle of the 20th century.
Interference Contrastor, as in the German Leitz objectives seen here,
Interf. Kontrast. There is a manual available for the use of the Leitz apparatus on this site.
Interference Contrastbecause they have built-in prisms. In addition to strain-free objectives, NIC requires the use of a (NIC) condenser, AND also a NIC intermediate tube containing an adjustable prism, placed between the binocular head and the optical tube. Shown to the left is an olympus combined phase contrast and NIC condenser, along with a typical Olympus 20X S-plan PL objective, that can be used for both phase contrast and DIC work. Note the condenser has an N.A. of 1.4, suitable for high power homogenous oil immersion work. Unlike phase contrast, DIC images have a color imparted to them and also appear three dimensional. The color can be varied by the user to optimize contrast for the particular subject. An Olympus Vanox microscope equipped for NIC as well as phase contrast, is on this site.
modulatoris placed the rear focal plane of the objectives. Such objectives are labeled with either HMC or the words Hoffman Modulation Contrast spelled out, or simply Modulation.
uncoveredposition, or using objectives designed solely for uncovered use; such objectives became known as
metallographic objectives.
Epi-illumination systems were first developed by Zeiss in the early 20th century. In this type of illumination a directed and focused beam of light surrounds the objective and is arranged such that the optimal plane of illumination conincides with the focusing plane of each objective. These objectives direct highly oblique light on the subject to provide dark ground illumination to opaque objects. These objectives provide a concentric path around the outide of the objective optics that receive the image. An example of a Wild Epi objective is shown here to the left. Note it is much wider than a standard objective because of the accessory light path outside the optics that receive the image.
Minscribed on their barrel, as in the M-plan 40X objective by olympus seen to the left. The previously mentioned technique of DIC was also adapted to vertical illumination. A DIC vertical illumination Olympus Vanox microsccope is in this collection and will be featured on this site in the future.
INSCRIPTION | USUAL MEANING |
---|---|
immersion | water immersion |
wet or dry | dry or water immersion |
oil | oil immersion |
gl gli or gly | glycerin (=glycerol) immersion |
w | water |
HI | homogenous immersion (=oil immersion, including condenser) |
xxxo | angular aperture in degrees |
B.A. | balsam angle (=angular aperture in canada balsam) |
n.a. or N.A | numerical aperture |
Fl | Flourite |
PL | Plan (flat field) |
NPL | normal field of view, flat(planar) |
Holos | Holoscopic (Watson) |
Tubus or T.L. | Mechanical Tube Length |
Achr or Achro | achromatic |
Para | Parascopic(Watson) |
Apo | Apochromatic |
planapo | flat field apochromatic |
THE FOLLOWING EXAMPLES ARE FOUND MAINLY OBJECTIVES DATING FROM THE 2ND HALF OF THE 20TH CENTURY: | INSCRIPTION | USUAL MEANING |
S or P or Po or Pol or SF | strain free, low birefringence for polarized light work) |
Fluotar | Flourite |
Ph, PHACO, PC | phase contrast objective |
DL,DM | phase contrast: dark low, dark medium |
PLL, PL | phase contrast: positive low low, positive low |
NL,NM,NH | phase contrast: negative low, negative medium, negative high contrast (higher contrast regions appear lighter) |
BD | brightfield or darkground |
EPI | episcopic (vertical) illumination |
F | focal length |
WD | working distance |
DIC or IC | differential interference contrast |
NIC | Nomarski Differential Interference Contrast |
Interf. Kontrast | interference contrast-usuall Smith type (German Notation) |
M | metallographic (without coverslip) |
Neo | Dark Ground vertical illumination(Zeiss) (not to be confused with Neofluar which are strain free high transmission objectives) |
U | Can be for ultraviolet light microscopy or alternatively for use with a universal stage |
UV | Ultraviolet transmitting objective |
DI, MI or TI | Interferometry, noncontact, mutliple bear (Tolanski) |
HMC | Hoffman Modulation Contrast |